This invention relates to measuring the dimensions of an elongated part, and more particularly to a measuring apparatus capable of accurately correlating the cross sectional measurements to the longitudinal location on the part. Additionally, this invention relates to a method for easily comparing measurement data to the designed dimensions of the part, thus greatly simplifying part inspection.
The need to produce elongated structural parts that closely conform to designed parameters occurs throughout the aerospace industry as well as in other industries. These parts may either be simple constant cross section extrusions or more complex compound contour cross sections that taper from end to end and have precise notch or tab features. Further, these parts may be made of metal, such as aluminum, of composites such as graphite-epoxy, or of other rigid materials.
Part measuring technology has evolved considerably and has produced several useful devices for measuring constant cross section parts. See, e.g., Kioke, U.S. Pat. No. 4,805,309. Systems have been developed which move a part past a dimensional sensor and measure the part's maximum dimension passing the sensor. See, e.g., Hofelt, Jr. et al., U.S. Pat. No. 3,775,854. However, a method for accurately measuring multiple dimensions of the complex shaped parts found in airplanes has typically been very labor intensive. Additionally, prior measuring processes for such parts have suffered from measuring inaccuracies that often exceeded the part's dimensional tolerances. Methods exist for measuring some dimensions of constant cross section parts, but a preferable system would precisely measure variations in cross section and correlate the measurements with the longitudinal position of the part.
For example, on a typical "I" section airplane wing stringer having a compound contour with tabs on the upper flange, the tabs must be located accurately from the stringer's end within a small tolerance. To check these dimensions, operators manually measured the tab location by first placing a pair of shape templates against the part tab and, using a flexible measurement instrument such as a tape measure, measuring the distance from the point at which these templates intersect to the end of the stringer. Because different people position the templates against the tab slightly differently, and because the tape measure only approximates the compound contour of the part, repeatability in measuring a model part was sometimes outside the part's dimensional tolerance. A better system would allow precise measurement along the part's longitudinal axis,irrespective of contour, and would allow precise, repeatable measurement of part cross sectional features correlated to longitudinal position along the part.
Recording the measured dimensions has typically required the operator to read an analog gauge and record the measurement. Some measuring systems have improved on this by creating a digital output. See, e.g., Wilke, U.S. Pat No. 3,875,667. Further improvement in measurement technology made possible converting analog displacements to digital pulses suitable for computer manipulation for part classification requirements. See, e.g., Hofelt, Jr. et al., U.S. Pat. No. 3,775,854. A superior system would measure the member, create a digital record of the part measurements, and then graphically display the measured dimensions in a visual context such as a video monitor allowing even new users to immediately identify out of tolerance parts, and also allow the user to easily locate dimensional discrepancies on the part.
Thus, it would be desirable to create a measuring system that could continuously measure the cross sectional dimensions of a part in relation to its lengthwise position. Additionally, it would be very useful if the measured dimensions could be both graphically represented in relation to the part's desired dimensions in a user friendly format on a video monitor, and also stored for future reference.